Mixed signal integrated chips are driving increasingly higher performing applicants. Testing the chips that drive the computer systems require a wide range of escalating speeds, pin counts, analog bandwidth, resolution and timing requirements. Automated Test Equipment (ATE) ideally provides the accuracy and test coverage required for design verification while delivering high throughput and low cost for testing required in high volume manufacturing.
With the availability of more advanced chip design processes the sophistication of mixed-signal devices continues to evolve rapidly toward increased functionality and higher bandwidth. In conjunction, testing requirements are pushed toward higher speeds and greater precision in order to meet the new and evolving device test requirements.
In particular, differential signaling is used for high speed data transfer such as 3.2 Gbps or more. Differential signals can use lower voltage swings than single-ended signals. This is possible because the differential threshold in a differential receiver is better controlled than the threshold of a single transistor. The lower swing leads to faster circuits and can reduce power consumption. Differential signaling also reduces electromagnetic interference (EMI) since the opposite current carried on the two traces leads to cancellation of the electric and magnetic fields at larger distances. Similarly, differential signals are less sensitive to crosstalk.
In order to drive a differential signal, an output stage of a conventional differential pin driver 100 employs a class-A configuration with a constant current source 110, a switch 120, and two 50 ohm source resistances, as shown in PRIOR ART FIG. 1. A high voltage level signal (VIH) determines the driver output high level, and a voltage swing signal (VIS) determines the driver output swing (i.e. the current source provides a current equal to VIS/50 ohms).
However, in the configuration of FIG. 1, the driver 100 can only provide two levels: drive-high level=VIH; or drive-low level=VIH-VIS. A high speed signal DHI controls the output logic to be either high or low for each output pin (IO_P or IO_N). The configuration of FIG. 1 is inadequate for testing mixed signal devices having multi-band instrumentation at higher speeds.
As such, what is needed is a differential pin driver capable of testing mixed signal devices with advanced analog instrumentation, including multi-band analog instrumentation.